The practice of integrating capacitance diodes (varactors) and bipolar transistors in a common semiconductor body is important, in particular, when fabricating voltage-controlled oscillators (VCOs). Fields of application of voltage-controlled oscillators (VCOs) which are usually fabricated using bipolar or BICMOS technologies are, for example, mobile radio at frequencies of around 900 MHz to 2.4 GHz and radar technology at frequencies of around 24 GHz. Another application is, for example, a distance radar apparatus for use in automobiles in a frequency band from 76 GHz to 81 GHz. A transmission signal for such radar instruments is generated by a voltage-controlled oscillator. Known distance radar instruments are implemented using expensive III/V semiconductor technologies on account of the high transmission frequencies required. Recently, however, it has been possible to significantly improve the performance of SiGe heterojunction bipolar transistors (SiGe HJBT), with the result that silicon-based bipolar and BICMOS technologies which have cut-off and oscillation frequencies of 200 GHz are also suitable, in principle, for implementing an automotive distance radar apparatus using the above-mentioned frequency range of 76 GHz to 81 GHz.
The implementation of such radar instruments requires microwave-frequency transistors having a transition frequency of more than 200 GHz and suitable varactors to be integrated in a common semiconductor substrate.
The collector of a bipolar transistor, for example an NPN radio-frequency transistor, is fabricated, in known methods, using a silicon epitaxial layer on a highly doped buried n+-type layer (subcollector). In this case, the width of the collector is determined by the thickness of the silicon epitaxial layer. In comparison with older slower transistors, a modern bipolar transistor having a transition frequency of more than 200 GHz requires a very flat collector, that is to say a very thin silicon epitaxial layer, in order to achieve these high cut-off frequencies. However, when using conventional fabrication methods to integrate the above-mentioned components (radio-frequency transistors, varactors and high-voltage transistors), a thin epitaxial layer limits, on the one hand, the emitter-collector and base-collector breakdown voltages of the high-voltage transistors and, in particular, also the capacitance range in which the varactor can be varied. An important characteristic variable for characterizing this range is the Cmax/Cmin ratio, that is to say the ratio between the maximum achievable capacitance of the varactor and the minimum achievable capacitance. The greatest possible frequency range in which a VCO constructed with the aid of the varactor can oscillate presupposes a correspondingly large Cmax/Cmin ratio of the varactor. A bandwidth of the VCO of 12 GHz or more is desirable for expedient use in an automotive distance radar apparatus.
When integrating transistors and varactors in a common semiconductor substrate using the same fabrication process some conflicts of objectives emerge: in order to achieve the highest possible transition frequency in the case of radio-frequency and microwave-frequency bipolar transistors, the epitaxial layer which determines the collector width must be very thin. In the case of varactors, the thickness of the epitaxial layer determines the width of the cathode zone in which the space charge zone propagates, on which the achievable capacitance depends directly. A very thin cathode zone would greatly reduce the achievable Cmax/Cmin ratio, and thus the achievable bandwidth of a VCO constructed with the aid of the varactor would thus be greatly restricted.
There is a similar conflict of objectives when additionally integrating high-voltage bipolar transistors. In order to achieve the requisite electric strength (i.e. withstand voltage), the epitaxial layer would have to be selected to be considerably thicker than in a radio-frequency transistor in order to increase the collector width and thus to increase the electric strength of the collector zone.
For these and other reasons, there is a need for the present invention.